They do it for the same reason a kid cries. To get attention, to escalate a situation,to get a response.
I work in a customer facing environment and i have had plenty of people ranting in my face. If you just say nothing and deadpan them, they fairly quickly just burn themselves out and then return to normal. Sometimes they even apologise and put the kettle on ..hahaha,, The thing not to do is flinch,react,shout back because it will likely escalate possibly to an attack.